Methods for measuring mass flow rate of grain in a harvester have been used to provide a grain flow rate signal which can be used to calculate either the total mass of grain harvested within a given field area or the instantaneous yield of the crop at the present location of the harvester in the field. These data are useful to the agricultural producer to allow measuring the effect of different soft conditions or crop growing practices on crop yield. Total mass of grain is calculated by integrating grain mass flow rate versus time. Instantaneous crop yield is calculated by dividing instantaneous grain mass flow rate by the instantaneous rate at which the harvester is harvesting the field area.
Numerous methods have been used to measure mass flow rate of grain on harvesters. Co-pending U.S. patent application Ser. No. 07/716,293 discloses means and methods for measuring grain mass flow rate at the exit of a paddle type chain conveyor, by measuring the force resulting from grain striking an impact plate as grain exits the conveyor. Other methods, such as measuring electrical or electromagnetic properties of grain passing by or contacting a means for sensing said properties have also been used.
It is desirable to use a sensor which produces an output signal which is substantially linearly proportional to the value of the measured parameter. With such a sensor, calibration to establish accurate estimated values of the measured parameter requires only establishing the baseline output of the sensor that exists at zero value of the measured parameter, and the gain or scale factor, which is the ratio of a change in sensor output to a change in the measured parameter, of the sensor. Since gain is a constant value due to the linear input/output relationship, it can be established by measuring the output signal of the sensor at any value of the measured parameter.
However, in some measuring applications, it is not possible or practical to build a sensor which produces a highly linear input/output characteristic. In such cases, a sensor with a non-linear input/output characteristic must be used, which greatly increases the difficulty of accurately calibrating the sensor. Since the input/output characteristic of a non-linear sensor is a curve, and not a straight line, many more calibration constants are required to accurately describe the shape of the curve. Further, accurately establishing the shape of the curve requires measurement of the output signal of the sensor at many values of the measured parameter.
In some measuring systems, such as those for measuring grain mass flow rate on harvesters, identical sensors used in different physical installations, such as different machine models, or with different operating conditions, such as different grain types, have different non-linear input/output characteristics. The cost to perform tests to measure input/output characteristics for all combinations of machine models and grain types would be enormous, and makes such an approach impractical. Further, sensor input/output characteristics for a given machine model may vary from machine to machine due to slight variations in mechanical dimensions. It is therefore desirable to establish a method whereby an accurate input/output characteristic can be determined for a given sensor using values of a parameter which is easily measurable during actual field operation. On a harvester utilizing a mass flow rate measurement system which integrates grain mass flow rate versus time to obtain accumulated masses of separate loads of grain, said loads can be weighed on scales to determine their actual mass, after they are unloaded from the harvester onto a transport vehicle such as a truck or a wagon. Since load masses are the only measurements which are readily available during field operation of a harvester, it is desired to have a method to use these actual load masses to establish the input/output characteristics for a given sensor, harvester and grain type.
In prior art systems for measuring grain mass flow rate on a harvester, such at the system disclosed in co-pending application Ser. No. 07/716,293, the non-linear input/output characteristic for the grain mass flow rate sensor is represented by either an equation of a curve, such as a fourth-order polynomial, or by a series of straight line segments which approximate the curve with sufficient accuracy to limit approximation errors to small values. During a harvesting period when a load of grain which is to be weighed is harvested, the total accumulated mass of grain is computed by calculating grain mass flow rate at regular time intervals such as 1 second, based on the average grain impact force measured during each time interval, and integrating calculated grain mass flow rate over time as grain is harvested. After the load of grain is transferred from the harvester and is weighed, the actual mass obtained can then be used to calculate an adjustment factor which can be multiplied by the calculated load mass to obtain the actual load mass. This adjustment is equivalent to adjusting the values of the non-linear input/output characteristic curve for the sensor by this same adjustment factor at all points on the curve. If the initial shape of the non-linear curve is correct, this adjustment provides the correct non-linear curve which can subsequently be used for accurate calculation of grain mass flow rate during subsequent harvesting periods. However, if the initial shape of the curve is not correct, the adjusted curve will not accurately represent the actual input/output characteristic curve and will not produce accurate calculation of grain mass flow rate at all levels of mass flow rate. If actual masses are obtained for several loads of grain for which separate masses are calculated, and if the initial estimated shape of the non-linear curve is incorrect, different adjustment factors will be calculated for each load if the occurrences of mass flow rate levels are distributed differently for each load of grain, which is the usual ease. These adjustment factors can be averaged to obtain an average adjustment factor, to minimize errors due to an incorrect estimated shape of the characteristic curve, but accuracy is still limited by inability to accurately determine the actual shape of the curve during field harvesting. It is therefore desired to establish a practical method of accurately establishing the shape of the actual non-linear input/output characteristic curve of the grain mass flow rate sensor during field harvesting.